This invention relates to a color image forming apparatus which forms a toner image on an image carrying member by electrophotography and transfers it onto a recording sheet to obtain a color image, and more particularly, to a color image forming apparatus such as a printer, a copier or a facsimile capable of forming a color image with excellent color reproducibility onto a recording sheet such as OHP film (transparent film for overhead projector: synthetic resin recording sheet).
In the conventional color image forming apparatus, when a color image is formed on a recording sheet or particularly, thick paper or OHP film, it is necessary to melt the toner almost completely and smooth the toner surface by eliminating roughness, thus fixing conditions are changed each time a color toner image is fixed.
As a method to change fixing conditions, a method to change fixing temperature and a method to change fixing speed are available. The former method to change fixing temperature has the following problems; (1) It takes a long time to reach a specified temperature, (2) an offset due to a temperature change is likely to occur, further (3) color reproduction is insufficient by only raising temperature. On the other hand, the problems of the latter method in which the fixing speed is changed are that the distance from a transfer means to a fixing means needs to be longer than the length of a recording sheet, and that the fixing speed can not be reduced only to the extent that the trailing edge of a preceding recording sheet does not overlap the loading edge of a following recording sheet at the time of continuous image forming.
Anyhow, it is difficult to obtain a color image with excellent color reproducibility in image forming by methods to change fixing temperature or speed.
Considering these problems, the applicant has proposed a color image forming apparatus capable of obtaining a color image with excellent color reproducibility by transferring a recording sheet which has been fixed once to be fixed again by means of a recording sheet carrying means, in Japanese Patent Laid Open No. 2-141767.
However, because the image forming apparatus disclosed in the Japanese Patent fixes an image twice by means of the fixing means by means of a guide pawl, it has such problems that the structure is complicated and the unit size cannot help but become enlarged.